Jean-Jacques d'Ortous de Mairan

Jean-Jacques d'Ortous de Mairan
Engraving by Simon Charles Miger after Charles-Nicolas Cochin.
Born	26 November 1678 Béziers
Died	20 February 1771 (1771-02-21) (aged 92) Paris
Nationality	French
Fields	geophysics, astronomy, chronobiology

Jean-Jacques d'Ortous de Mairan (26 November 1678 – 20 February 1771) was a French geophysicist, astronomer and most notably, chronobiologist, was born in the town of Béziers on 26 November 1678. De Mairan lost his father, François d'Ortous, at age four and his mother twelve years later at age sixteen. Over the course of his life, de Mairan was elected into numerous scientific societies and made key discoveries in a variety of fields including ancient texts and astronomy. His observations and experiments also inspired the beginning of what is now known as the study of biological circadian rhythms. At the age of 92, de Mairan died of pneumonia in Paris on 20 February 1771.

De Mairan attended college in Toulouse from 1694–1697 with a focus in ancient Greek. In 1698 he went to Paris to study mathematics and physics under the teachings of Nicolas Malebranche. In 1702, he returned home to Béziers and began his lifelong study of several fields, most notably astronomy and plant rhythms. Furthermore, during his time in Béziers, he ate every day with Cardinal de Fleury; later, de Mairan founded his society under the protection of Cardinal de Fleury. Eventually, de Mairan received official lodging in the Louvre where he remained pensionnaire until 1743 and served as secretary from 1741 to 1743. In 1746, he was reinstated as pensionnaire geometre, or full-time boarding surveyor. It is reported that the Prince of Conti and other great lords heaped extravagant gifts upon him. He was also secretary to the Duke of Orléans.

In 1729, de Mairan performed an experiment that demonstrated the existence of circadian rhythms in plants, specifically the Mimosa pudica. He was intrigued by the daily opening and closing of the heliotrope plant and performed a simple experiment where he exposed the plants to constant darkness and recorded the behavior. De Mairan's key conclusion was that the daily rhythmic opening and closing of the leaves persisted even in the absence of sunlight. However, de Mairan did not infer that heliotropes have internal clocks driving leaf rhythms, but rather that they were able "to sense the Sun without ever seeing it". The concept of an internal clock was actually not formulated until much later, although de Mairan did suggest that "it would be curious to test [...] whether, using kilns heated to higher or lower temperatures, one could artificially recreate a day & night perceptible to [plants]; and whether in doing so one could reverse the order of the phenomena of true day & true night."